A grease typically is a solid or semi-liquid substance which includes a thickening or gelling agent in a liquid carrier. Almost any oil having lubricating properties may be used with a suitable gelling agent to make a grease. Petroleum oils, fatty oils such as castor oil, polyalkylene glycols, synthetic diesters, silicones, fluorocarbons and polyphenyl ethers are all found in commercial lubricating greases. (See, e.g., Braithwaite, "Lubrication and Lubricants," Elsevier Publishing Co., 1967, pages 181-183 and 223-228). The gelling agent frequently used in greases include fatty acid soaps, clays, silica, organic dyes, amides, and urea derivatives. The gelling agent forms a network structure in which the carrier is held by capillary forces.
When a low stress is applied through a sample of grease, the material acts substantially like a solid. If a stress above a critical value is supplied, the material flows and viscosity decreases rapidly. This decrease in viscosity is largely reversible since it is typically caused by the rupture of network junctions between filler particles, and these junctions can reform following the removal of the source of the stress.
A desirable property of the grease is the absence of syneresis, i.e., its ability to maintain uniform consistency. Generally, syneresis is controlled by assuring dispersion of an adequate amount of the gelling agent throughout the composition.
Grease compositions have been used as cable filling material in both conventional (electrical) communications as well as for optical fiber cables. U.S. Pat. No. 4,701,01,5 discloses grease compositions comprising an oil, a gelling agent such as colloidal particles of silica or clay, and optionally a bleed inhibitor. The oils which are disclosed as being useful in the compositions of that patent include certain naphthenic or paraffinic oils having certain specific gravities and properties, polybutene oils of similar specific properties, triglyceride based vegetable oils, polypropylene oil, chlorinated paraffin oils, and polymerized esters. The colloidal particle filler material preferably comprises silica particles such as fumed silica or precipitated silica. Preferred bleed inhibitors include styrene-rubber, styrene rubber-styrene, or other block polymers, and/or semi liquid rubbers such as high viscosity polyisobutylene. Other ingredients such as thermal oxidative stabilizers optionally may be present.
One disadvantage of the compositions of the above cited patent is that relatively large amounts of silica must be included to meet the appropriate service requirements for the grease. Thus, relatively stiff greases are obtained which are not entirely suitable for such fiber optic cables. In particular, such stiff greases will affect attenuation (microbending) of the fiber optic cable at relatively low temperatures (i.e., below about 50.degree. F.). Furthermore, if the amount of silica is reduced to lower the viscosity of the grease, the shear characteristics of the formulation are reduced to a value which is less than desirable. Moreover, such a reduction in silica often results in syneresis.
U.S. Pat. No. 4,839,970 to Goetze et al. discloses a filling compound that can be utilized to fill light waveguide leads or light waveguide cable cores. The filling compound comprises polypropylene glycol to which a second compound chosen from the group consisting of silica, aluminum hydroxide, aluminum oxide, a halogen containing organic compound, or mixtures thereof, is added as a thixotropic agent. The filling compound consists of between approximately 50 to about 99 weight percent of polypropylene glycol and between approximately 50 to about 1 weight percent of the second compound. Finely distributed fumed silica that acts as a thixotropic agent is used as the second compound, and the polypropylene glycol has an average molecular weight of between approximately 2000 to about 3500. In a further embodiment, the filling compound includes an antioxidant which is present in an amount of approximately 0.1 to about 1 weight percent of the filling compound.
While the greases of that patent can be used to seal optical cable, they are incapable of meeting current industry specifications which require improved performance with respect to bleed resistance and oxygen inhibition resistance.
Accordingly, a need exists for novel polyol containing grease compositions which are capable of providing an optimum balance of viscosity and shearing characteristics such that the compositions are suitable for use with fiber optic cables even at lower temperatures. In addition, these formulations must provide properties which are capable of passing the appropriate drip tests, paper bleeding tests, and syneresis requirements for such cable filling greases.